import cv2
import pylab
import numpy as np

img = cv2.imread('train_image/img.png')

img[600:800, 1000:1200] = np.random.randint(0, 256, (200, 200, 3))

roiImageRgb = cv2.cvtColor(img, cv2.COLOR_BGR2RGB)

pylab.imshow(roiImageRgb)
pylab.show()